1. Field of the Invention
This invention relates to the mechanical and environmental protection of cable joints and other elongate substrates, and more particularly to an assembly comprising a wraparound liner and a recoverable sleeve, and to a method of protecting a cable joint which utilizes such an assembly.
2. Discussion of the Art
Certain cables may require additional mechanical and environmental protection, especially in joint areas, if they are to be reliable over long periods. In joint areas, protective layers may have been stripped away in order to make the joint, and ordinarily must be replaced by protection equally as good. This is generally true whether the cables are current-carrying (i.e., power distribution of transmission, telecommunication, or cable television), optical fiber, or other. This discussion applies to the jointing of all types of cables where joint cases may be used, although for the sake of clarity, it will focus on current-carrying cables.
Current-carrying cables generally comprise one or more conductors which may be of solid metal or of stranded construction, and which are typically of copper or aluminum. They are insulated from one another by materials such as oil impregnated paper or polymers, which may be cross-linked to give then superior properties. Individually insulated conductors are known as cores. The type and amount of insulation is determined largely by the voltage rating of the cable in the case of power cables or by the need for mechanical strength in the case of telecommunication cables.
The core insulation may be surrounded by one or more sheaths. Oil impregnated paper insulation, used for some power cables, is typically surrounded by a metal sheath, generally of lead or aluminum, to protect the insulation from moisture. Polymer insulated power cables may require the additional insulation provided by one or more polymeric sheaths generally applied by extrusion.
The core or cores may be surrounded by a layer of metal armor, screen and/or shielding in the form of wire, sheet, mesh, screen or tape, generally of steel or aluminum, which may provide a path to ground and carry fault currents. Armoring in addition provides longitudinal strength and mechanical protection, especially desirable for buried cable. For some applications, these metal components may be covered by or embedded in a compound to prevent the ingress of moisture. High voltage power cables may have each core surrounded by its own screen or shielding, but there is usally only one metal armor layer positioned near the outer protective jacket.
An outer protective jacket typically provides further electrical, mechanical and environmental protection, and polymer layers such as polyvinyl chloride are commonly employed.
A preferred joint enclosure for telecommunication cables is a recoverable polymeric sleeve, preferably a heat-shrinkable polymeric sleeve. Polymeric heatrecoverable materials and articles formed therefrom are disclosed in U.S. Pat. Nos. 2,027,962 to Currie and 3,086,242 to Cook et al, both of which are herein incorporated by reference. Recoverable articles which are recoverable without heat are disclosed in U.S. Pat. Nos. 4,070,746 to Evans et al, 4,135,553 to Evans et al and 4,179,320 to Midgley et al, all of which are herein incorporated by reference.
These sleeves may be tubular, requiring positioning around one cable end prior to jointing and subsequent repositioning around the joint, or they may be of wraparound design, such as those disclosed in U.S. Pat. Nos. 3,455,336 to Ellis and 4,200,676 to Caponigro et al, both of which are herein incorporated by reference. A layer of mastic or adhesive may be interposed between the jointed cables and the sleeve to provide water-tight environmental sealing, especially desirable for buried cable. Wrapable sleeve enclosures are preferred for buried cable to minimize trench length and excavation costs.
An inner protective liner is desirably positioned around the cable joint before installation of the sleeve to provide additional mechanical protection. These liners generally bridge the joint and provide support for the recovered sleeve. Improved impact resistance, especially necessary for buried cables with rock backfill, is provided. Known liners may be of tubular design, or may be wraparound split-tubes, half-shells or sheet. A wraparound liner, suitable for a telecommunication splice case and comprising a fibrous sheet support and a vapour impermeable metal layer is disclosed in U.K. published application No. 2,059,873, the disclosure of which is incorporated herein by reference. A thermoplastic support layer with a foamed ploymeric heat barrier layer is disclosed in U.K. published application No. 2,069,773, the disclosure of which is also incorporated herein by reference. Sheet liners may have fold lines to facilitate wrapping, and crowned ends to facilitate neck-down onto the cables, as taught in, for example, U.S. Ser. Nos. 94,815 and 228,349.
Enclosures for joints in power cables are similar to those used for joints in telecommunication cables etc., although there are certain differences. Bitumenfilled metal or concrete molds were originally used for joint enclosures, later replaced by resin-filled boxes of steel, cast iron, lead or tinned copper, and today recoverable polymeric sleeves and liners are used, such as those disclosed in U.S. Pat. Nos. 4,142,592 to Brusselmans and 4,282,397 to Siedenburg et al, the disclosures of which are herein incorporated by reference. The cables will generally have a metal sheath, screen, shielding, or armoring, which should be interconnected across the joint to provide continuity of shielding or of mechanical protection or to provide a fault current path. A metal box, metal liner, metal braid or metal tape may provide the desired electrical continuity.
Power cables generate a significant amount of heat in operation, and the conductor of a polymer insulated power cable, for instance, may reach a temperature in normal operation of 70.degree. C., be rated for maximum continuous operation of 90.degree. C. and have a short-term rating of 130.degree. C. Heat transfer from the cable to the environment can become critical, since insulation degradation is generally an exponential function of temperature. A liner which comprises a generally solid-wall canister can, therefore, under certain circumstances have the double disadvantage of unnecessary bulk and the poor heat transfer that results from the entrapment of an insulating layer of air. The only satisfactory way of avoiding the overheating that follows, is to fill the canister with a heat conductive potting compound.